The present invention relates to an electrode and conductor arrangement for a three-phase electric arc furnace whose electrodes extend vertically or obliquely downwardly into the furnace and are fastened to electrode support arms.
Each of the electrode support arms of a known furnace of this type holds a group of conductors which are connected to the associated electrodes. The conductors connected to the outer electrodes, when seen with respect to the longitudinal axis of the conductors, are arranged perpendicularly on top of one another, while the conductors connected with the center electrode are arranged in the center thereof and axially symmetrically thereto as well as closely above one another.
It is known that electric arc furnaces, for example those used in steel manufacture or for reduction processes, must essentially be able to provide symmetrical current input with substantially equal distribution of the current load to the conductors or conductor parts, and low inductance levels. In addition, they should allow for the smallest possible deviations in height of the current paths, so that their influence on the distribuion of inductance is minimal, and the lowest possible spatial height for the high current paths is maintained.
An asymmetrical current input to an electric arc furnace is associated with unequal, mutual inductive influences which are produced between the high current loops formed by conductors, also called strands, of such a three-phase current system, which usually presents low resistances. It has therefore previously been proposed to attempt to obtain an arrangement whose cross section is as axially symmetrical as possible in the three conductor strands, i.e. to effect a so-called triangulation.
Moreover, there are inductive influences between the conductors, or conductor sections, respectively, in the strands, or high current paths, respectively. The conductor sections carry respectively different current levels thus correspondingly different current densities, resulting, inter alia, in different thermal loads and, finally, higher total losses in the leads. In known systems, the current load differences between conductor sections sometimes reach a ratio exceeding 2:1.
Thus, it is an object of the present invention to optimize the conductor configuration of the conductor sections as well as of the conductor strands in such electric arc furnace.
It is further known that an increased furnace output requires a substantial reduction in the inductances of the high current power leads particularly since an increased output is preferably achieved by increasing the electric arc current intensity. The resulting lower resistance has a considerable influence on the line inductances so that without countermeasures an increased voltage requirement and worsening of cos .phi., the power factor, would have to be accepted.
However, every change in the geometry of the current paths of a low-resistance system has a noticeable influence on the distribution of the inductances.
Thus, for example, in electric arc furnaces employing scrap metal for the manufacture of steel, the height levels of the electrodes must be varied considerably during the melting phase so that during this operating phase there inevitably occur deviations in inductance compared to the values for a symmetrical configuration. However, during operation with a calm bath, deviations in height of the supporting arms carrying the conductors cannot be avoided as the electrode mounts should not be attached in the region of the electrode nipples which, as experience has shown, may lead to considerable difficulties.
For example in a triangulated arrangement, these changes in height result in asymmetry of the inductances, with the consequence of a correspondingly asymmetrical furnace operation.
It is therefore a further object of the invention to provide an arrangement in which the deviations in height which result inevitably during operation, and particularly those which result from different attachment of the electrodes, with respect to the theoretically prescribed desired clamp-in length, have the smallest possible effect on the inductance distribution.
A further object of the invention is to limit the spatial height of the high current paths to a minimum value.
It is known in the art that the high intensity current paths of three-phase current electric arc furnaces, including their counterinductances, can be represented by an equivalent circuit diagram of a system with decoupled self-inductances. Consequently, the sum of two equivalent circuit inductances of two strands is equal to the self-inductance of the high current loop formed of the two strands. A lower loop inductance, however, can be achieved by arranging the lines in such a way that the conductors of the three strands lie as close together as possible and the conductors of one strand are disposed vertically above one another. Since, with three juxtaposed strands of the same geometry, it is known that the equivalent circuit inductance of the center strand is always less than that of the outer strands, the above-mentioned spacing rules can be used to realize small system inductances, particularly for the two outer strands.
Accordingly, it has been proposed, as disclosed in German Offenlegungsschrift [Laid-open Application] No. 1,806,504 to arrange preferably three conductors vertically on top of one another for the respective outer electrodes, with the group of conductors associated with one of the outer electrodes being parallel to the other one, and, if necessary, to place the center conductors as close together as possible.
However, the disclosure of that application is limited to the arrangement of the connections of the conductors to the electrode support arms, and does not include any discussion of the succeeding connection to the transformer output, which in part consists of flexible lines, nor about the limits within which the distance dimensions of the individual conductors can vary.
A similar disclosure is provided in British Pat. No. 975,651.
The above and other objects are achieved, according to the invention, in an electrode and conductor arrangement for a three-phase electric arc furnace supplied with power from a transformer, including three electrodes which extend downwardly into the furnace, supporting arms supporting the electrodes, and a plurality of current conducting paths each composed of a rigid conductor, which is fixed to a respective supporting arm and is conductively connected to a respective electrode, and a flexible conductor connected in series between the rigid conductor and the transformer, the rigid conductors extending parallel to one another, the electrodes being spaced from one another in the horizontal direction transverse to the longitudinal axes of the rigid conductors such that two of the electrodes are outer electrodes and the third electrode is a center electrode disposed between the two outer electrodes, a respective plurality of conducting paths being conductively connected to each electrode, the conductors of all conductive paths associated with the same electrode being vertically spaced from one another, the vertical spacing between the conductors associated with the center electrode being less than that between the conductors associated with the outer electrodes, and the conductors associated with the center electrode being positioned between, and axially symmetrically relative to, the conductors associated with the two outer electrodes, by spacing the conducting paths associated with the center electrode a minimum distance, in the horizontal direction transverse to the longitudinal axes of the rigid conductors, from the conducting paths associated with each outer electrode, providing only two conducting paths for each outer electrode; and one or two conducting paths for the center electrode causing the vertical spacing between the two conducting paths associated with each outer electrode to be greater than the maximum spacing between the actual positions of the electrodes and the positions thereof required to achieve electrical symmetry and associated with identical distances between the lower end of each electrode and its point of connection to its associated rigid conductors, and making the distance between the center electrode and the transformer greater than that between each outer electrode and the transformer, for causing the current flow in the arrangement to be electrically symmetrical and minimizing the line inductances of the conducting paths.
The invention thus advantageously relates not only to the conductor arrangement at the electrode supporting arms but also to the electrode arrangement and the conductor arrangement over the entire path from the electrode to the transformer. In particular, the high intensity current paths are each limited to two conductors, which has the advantage over the six-pole arrangement following the supporting arm in German Application No. 1,806,504 that it is much less complicated, provides greater play for the individual conductors during unavoidable or necessary vertical movement of the conductor system and results in a significant reduction of the system inductances or retention of the reduced inductance realized by other measures.
Additionally the arrangement according to the invention includes all parts of the high intensity current paths in achieving the objects of the invention. Preferably, embodiments of the invention utilize a star or delta connection in the transformer or directly at the transformer terminals with fixed geometry and minimum spaces between conductors and lengths of the conductors in the flexible three-pole connection.
Particularly in order to meet the requirement for minimum change in inductance when there is a change in the height of the supporting arms, current is conducted over only two conductors or conductor bundles per strand which, additionally, are spaced apart vertically by a constant distance along their lengths. At the outer electrodes, the vertical distance between the conductors is greater than the maximum distance by which the electrodes, due to unequal consumption and/or technically required attachment, are shifted out of the position which is required with respect to electrical symmetry and which is characterized by identical clamping length for all electrodes. Preferably, the vertical distance, or spacing, between the rigidly held conductors should be about two to three times as large as the maximum distance defined above by which the clamped-in electrode lengths, i.e. the path from the contact jaw of the electrode to its tip, differs from the theoretical optimum length.
Shifting the respective outer conductors further apart results in no improvement in the total inductance value or in the change in inductance, but does contravene the requirement for the lowest possible structural height of the three-phase electric arc furnace. If the spacing between the outer conductors is selected, for example, to be only of the same magnitude as the deviation of the electrode clamping which differs from the optimum position in view of the nipple, the maximum reactance asymmetry may double approximately, compared to the preferably proposed double to triple distance, i.e., for example, a distance 2.5 times as great.
The same effect can be realized for the flexible conductor bundles in the outer strands with only half the vertical spacing compared to the rigid conductors on the supporting arms since the mean vertical displacement between the flexible conductors is only half as great as the vertical displacement between the rigid conductors disposed on the supporting arms.
According to particular features of the invention, if there are deviations in the height of the supporting arms with respect to the optimum position within the maximum permissible values described above, the maximum degrees of asymmetry as they occur with unfavorable relative positions of the supporting arms can be reduced further in that the equivalent circuit inductance of the conductors of the center elctrode is increased by about 4 to 6% with respect to the value which produces symmetry in the normal position of the supporting arms. The maximum asymmetry within the given range is thus reduced, more advantageously, according to theoretical calculations, to about 4/5.
Finally, the structurally greatest possible horizontal approximation of the two outer conductor pairs, which is desirable due to the requirement for a reduction of the equivalent circuit inductances of the outer strands, and optimum vertical spacing between the conductors of one electrode, with which small changes in inductance due to changes in height and a minimum structural height are realized in the dimensioning of a furnace, can bring the result that with the conventional arrangement of the electrodes the equivalent circuit inductance of the center conductors will not reach the value of the two outer conductor pairs and the three-phase system will again become electrically asymmetrical. This difficulty can be overcome by the present invention in that, instead of the conventional electrode arrangement in which the center electrode is held at a supporting arm which is shorter than those holding the two outer electrodes, an arrangement is used in which the center electrode is held by a longer supporting arm which is brought through between the two outer electrodes.
Advantageously the locations where the electrodes are clamped to the supporting arms lie at equispaced points in a horizontal plane and always at the same distance from the vertical longitudinal axis of the furnace. The described arrangement has two advantages: firstly, the symmetry of the three-phase system is re-established by this measure; secondly, the equivalent circuit inductances of the outer conductors are reduced by a further degree.
The unavoidable differences in height with respect to the normal height position are known to produce additional inequalities in the distribution of the currents to the conductors or bundles of conductors connected to the electrodes. This can be avoided by vertically crossing the conductors of each electrode. An optimum crossover point lies in the vicinity of the connecting point between the flexible conductors and the rigidly arranged conductors on the supporting arms for each electrode, where such crossing can also be effected particularly favorably from a structural point of view.
A further similar cross-over point is the point of connection between the flexible conductors and the rigid conductors on the transformer side. This is appropriate if, for structural reasons, the center rigid conductor tube length on the transformer side is substantially greater than the vertical spacing between the outer conductors.
Advantageously, the vertical offset produced by the crossing of the conductors due to the different height arrangement of the flexible conductors is compensated by the introduction of an extension piece of corresponding length which is fastened to the lower rigid conductor. According to a further feature of the invention, the lower flexible conductor of each pair, which is connected with the upper rigid conductor, is supported by the lower rigid conductor, e.g. on the supporting arm of the respective electrode, via an insulating piece.
The requirement for uniform current distribution to the conductors connected to an electrode is automatically met for the normal height position of the supporting arms with respect to the rigid conductors if the point of connection between a rigid conductor and its associated flexible conductor is an electrically conductive one and, additionally, if the conductor or conductors for the center electrode are precisely centered horizontally as well as vertically with respect to the conductors of the two outer electrodes. Because of the curved paths followed by the flexible conductors, this position, which is called the normal height position, also does not produce equality in the current loads for the flexible conductors so that breaking or eliminating the electrically conductive connections at the above-mentioned connection points produces a current distribution which results in less unequal distribution in the entire system.
In conventional power lead arrangements the tubes making up a conductor run are joined to one another at the ends at which the flexible conductors are connected. In this way both the flexible conductors and the tubes of each run form galvanically closed current loops. If the connection from the lower to the upper tube is cut and the current path is crossed over at the tube--flexible conductor junction so that only one single current loop is obtained for each conductor run from the connection of the flexible conductors on the transformer to the electrode holders, then current distribution via the groups of conductors becomes considerably more uniform. The current then flows from the lower flexible conductor to the upper tube and from the upper flexible conductor to the lower tube of a phase.